1. Field of the Invention
The present invention relates to a cable television (CATV) receiver system of a double conversion method for receiving CATV broadcasting signals, and in particular, to an automatic fine tuning (AFT) circuit for keeping at a predetermined value a frequency of a second intermediate frequency signal delivered from a tuner of the CATV receiving system.
2. Description of the Prior Art
In general, in a CATV receiver system (for example, a CATV converter) for receiving the CATV broadcasting signals, there is adopted a double conversion method to prevent the unnecessary emission of signals from a local oscillator and to attain various performances of a tuner (front end) section.
FIG. 4 is a block diagram showing a configuration of a CATV receiver system of this kind. In this configuration, a receive signal (frequency fRF) selected by a tuning circuit (not shown) from high-frequency signals supplied via a coaxial cable from a broadcasting station is supplied to a first mixer M1 of the tuner 1 so as to be mixed with an oscillation output (frequency fosc.sub.1) of a first local oscillator L1, thereby producing a fist intermediate frequency signal (frequency fIF.sub.1 =fosc.sub.1 -fRF). Subsequently, the first intermediate frequency signal is mixed in a second mixer M2 with an oscillation output signal (frequency fosc.sub.2) from a second local oscillator L2, thereby producing a second intermediate frequency signal (frequency fIF.sub.2 =fIF.sub.1 -fosc.sub.2). The first and second local oscillators L1 and L2 include voltage control oscillators (VCOs), respectively such that the oscillation frequency fosc.sub.1 of the first local oscillator L1 is set by means of a tuning system 2. The tuning system 2 includes a tuning button, a micro-computer, and the like to select a reception channel, so that when a reception channel is selected by the tuning button, a voltage necessary to attain the first local oscillation frequency fowc.sub.1 corresponding to a frequency of the selected reception channel is applied to a variable capacitance (variable capacity diode) functioning as a portion of the tuning capacitance of the first local oscillator L1. A second intermediate frequency outputted from the second mixer M2 of the tuner 1 is supplied to a demodulation circuit 4 via a band-pass filter 3 constituted with a surface acoustic wave (SAW) filter. The band-pass filter 3 has a characteristic to pass a voice intermediate frequency fIF.sub.2 s (41.25 MHz) and a video intermediate frequence fIF.sub.2 p (45.75 MHz), an adjacent channel interference prevention characteristic necessary to efficiently prevent an interference with an adjacent channel, and a characteristic such as the Nyquist strobe required to correct the frequency of a television signal transmitted in the vestigial side-band transmission system. The demodulation circuit 4 comprises a video detection circuit to detect a video signal component of the second intermediate frequency signal supplied via the bandpass filter 3 and a voice detecion circuit to detect a voice signal component thereof so as to respectively output video and voice signals of the base band from which the carrier signal is removed. The video and voice signals of the base band are demodulated by a demodulation circuit (not shown) into signals of a frequency band associated with a free channel of the television broadcasting in the VHF or UHF band so as to be delivered to an antenna terminal of a television receiver.
Incidentally, the conversion of the video and voice signals into signals of the base band is achieved for the following reasons. .circle.1 In a CATV system, to prevent nonsubscribers other than those who have paid the fee of the broadcasting from illegally televiewing the broadcasting, a scramble processing is effected on the side of the broadcasting station. On the receiver side, a descramble operation is accomplished in the base band to restore the original screen. .circle.2 In order to enable a sound volume adjustment by an operation on the CATV receiver system without necessitating the operation of the sound volume control on the television receiver side, the voice signal level is adjusted through the base-band conversion. .circle.3 The receiver system is enabled to obtain various data superimposed onto the television signals sent from the broadcasting station.
In the configuration above, when the oscillation frequencey fosc.sub.2 of the second local oscillator or the first intermediate frequency fIF.sub.1 shifts from the predetermined value, the second intermediate frequency fIF.sub.2 is changed and hence cannot correctly pass the hand-pass filter 3. Particularly, when the second intermediate frequency fIF.sub.2 is changed to a higher value, the video intermediate frequency fIF.sub.2 p (predetermined value 45.75 MHz) becomes to be beyond the band pass of the band-pass filter 3 even if the change is small. As a result, the demodulation circuit 4 cannot attain the normal video signal.
To prevent this disadvantageous phenomenon, there is disposed an AFT circuit 5 for continuously keeping at a predetermined value the second intermediate frequency fIF.sub.2 delivered from the tuner 1. The AFT circuit 5 detects the amount of fluctuation of the second intermediate frequency fIF.sub.2 from the normal value so as to supply the second local oscillator L2 with an oscillation frequency control voltage (to be referred to as an AFT voltage herebelow) VAFT proportional to the amount of fluctuation, thereby controlling the second local oscillation frequency fosc.sub.2. The AFT circuit 5 comprises an FM detection circuit 6, an AFT defeat circuit 7, and an AFT clam circuit 8. The FM detect circuit 6 achieves an FM detection on the video intermediate frequency signal of the second intermediate frequency signal so as to output as the AFT voltage VAFT a detection output voltage corresponding to the change of the video carrier frequency as shown in FIG. 5. The AFT defeat circuit 7 is disposed to invalidate the AFT operation in the channel select operation for preventing a wrong operation, and in an ordinary case, the the AFT voltage VAFT supplied from the FM detect circuit 6 is directly delivered to the AFT clamp circuit 8. On the other hand, during a defeat period of time beforehand set corresponding to a channel selection period of time required to be elapsed from when the channel selection button of the channel selector 2 is activated to when the oscillation frequency fosc.sub.1 is set, the AFT voltage VAFT is fixed to a constant defeat voltage Vd so as to set the second local oscillation frequency fosc.sub.2 of the second local oscillator L2 to a fixed value. Furthermore, the AFT clamp circuit 8 is disposed to establish the lower limit value of the AFT voltage VAFT and is constituted as shown in FIG. 6 with resistors R.sub.1 and R.sub.2 for dividing the power source voltage VB, a capacitor C.sub.1, and a diode D. Moreover, the clam voltage Vc developed at a connection point between the resistors R.sub.1 and R.sub.2 is expressed as follows. ##EQU1## Assuming here the voltage drop of the diode D in the forward direction thereof to be VF (about 0.6 V), the AFT voltage VAFT delivered via the AFT clamp circuit 8 cannot be equal to or less than Vc-VF. In addition, the AFT voltage VAFT supplied from the AFT circuit 5 constructed as described above is applied to a variable capacitor functioning as a portion of the tuning capacitance of the second local oscillator L2 so as to control the second local oscillation frequency fosc.sub.2.
In a case, for example as shown in FIG. 7, where the video frequency fRFp of the reception signal is 61.25 MHz and the voice or sound frequency fRFs thereof is 65.75 MHz, if the first local oscillation frequency fosc.sub.1 is set to 762.6 MHz, the voice intermediate frequency fIF.sub.1 s and the video intermediate frequency fIF.sub.1 p of the first intermediate frequency signal are attained as 696.85 MHz and 701.35 MHz, respectively. Furthermore, if the second local oscillation frequency fosc.sub.1 is set to 655.6 MHz, the voice intermediate frequency fIF.sub.2 s and the video intermediate frequency fIF.sub.2 p of the second intermediate frequency signal are attained as 41.25 MHz and 45.75 MHz, respectively. Moreover, the defeat voltage Vd outputted from the AFT defeat circuit 7 in the channel selection is set such that the second local oscillation frequency fosc.sub.2 develops a value (for example, 656 MHz) slightly higher than the predetermined value 655.6 MHz. As a result, when the AFT voltage VAFT increases, the second local oscillation frequency fosc.sub.2 increases and the second intermediate frequency fIF.sub.2 decreases; consequently, the video intermediate frequency fIF.sub.2 p of the second intermediate frequency signal is set to a value slightly lower than the predetermined value 45.75 MHz.
Incidentally, during a reception by means of the CATV receiver described above, when the transmission from a broadcasting station is temporarily interrupted due to a check of a transmission system of the station or a failure of a coaxial cable, the reception signal becomes to be missing and hence an AFT lockout takes place. The AFT lockout is a phenomenon in which the second intermediate frequency fIF.sub.2 greatly shifts from the normal value and becomes to be beyond the pass band of the band-pass filter 3, the AFT circuit 5 consequently cannot detect the amount of fluctuation of the second intermediate frequency fIF.sub.2, and hence the second intermediate frequency fIF.sub.2 cannot be corrected to the normal value. In addition, when the AFT lockout occurs, the receiver system is kept remained in the reception disabled state even when the transmission is restarted. Furthermore, a state where the AFT circuit 5 is not set to the loackout and hence the second intermediate frequency fIF.sub.2 can be corrected is called an AFT capture range, which is determined according to the pass band characteristic of the band-pass filter 3. As a cause of the AFT lockout, there may be considered a wrong operation of the AFT circuit due to a beat signal generated by a distortion or the like in the tuner 1 caused by a noise component or a multichannel input. In this case, the AFT circuit 5 ordinarily operates to lower the AFT voltage VAFT. Furthermore, the AFT lockout appears in a case where the channel selection time does not match with the defeat time beforehand set for the channel selection.
In order to effectively prevent the AFT lockout, it is only necessary to set the clamp voltage Vc of the AFT clamp circuit 8 to a higher value and the lower limit Vc-VF of the AFT voltage VAFT to a higher value. However, when the clamp voltage Vc is set to a higher value, the capture range of the AFT circuit 5 determined by the pass band characteristic of the band-pass filter 3 is narrowed, which leads to a problem that the system cannot cope with the total error (about 1 MHz in an ordinary case) including the errors of the first local oscillation frequency fosc.sub.1, the reception frequency fRF, and the second local oscillation frequency fosc.sub.2. Moreover, when the AFT voltage VAFT becomes to be similar to the clamp voltage Vc, a noise is superimposed onto the AFT voltage VAFT, which leads to a problem that a wrong operation is developed by the AFT circuit.